Bandpass filters (BPF) are commonly used in signal processing for various purposes. A BPF generally involves some form of resonator that stores energy in a given frequency band. This resonator will have an input coupling and an output coupling. Classical filters for electronic circuit applications are built on this principle. The resonator may be, for example, in the form of a transmission line cavity, waveguide cavity, lumped inductor and capacitor components, or a crystal where mechanical resonances of the crystal are coupled via electrodes to the electrical circuit. The crystal could also be in a form of a small block of ceramic material. An active form of a bandpass filter could include buffers associated with the input and output resonator couplers such that the external coupling does not degrade the frequency selectivity of the resonator. Such an active filter is illustrated in FIG. 1, which shows a generic active bandpass filter (BPF) having input and output buffers 102, resonator couplings 104, and a resonator 106.
The BPF of FIG. 1 can be made into a tunable BPF if the properties of the resonator can be adjusted. If they can be adjusted using passive elements, then the BPF is a tunable BPF. The energy storage of the resonator can also be arranged with feedback in which signal from the output coupling is fed back into the input coupling. This is shown in FIG. 2, which depicts a generic BPF with a feedback path 110. Referring to FIG. 3, a gain block 112 and delay block 114 may be added that condition the feedback to modify the resonance slightly. The addition of a gain block will turn a passive tunable BPF into an active tunable BPF. With this active feedback, more control is possible in which the phase and the amplitude of the feedback can be controlled to give a narrower bandwidth and finer control over the center frequency.
More specifically, the resonator feedback can be implemented in which the gain and the delay of the resonator feedback is assumed to be adjustable which modifies the frequency selectivity characteristics of the BPF. FIG. 3 shows control of the BPF feedback being implemented with the delay block 114, where the adjustability of the circuit elements is denoted by a diagonal arrow through the element.
If the overall loop gain (the loop consisting of the feedback path 110, couplers 104 and resonator 106) exceeds unity then the BPF becomes an oscillator, resonating at a frequency determined by the properties of the resonator 106 itself and the feedback loop 110. Backing off the feedback gain such that the loop gain is slightly less than unity results in a BPF with an arbitrarily narrow bandwidth. If the resonator 106 selectivity is reduced such that it has a broader pass band then the feedback can tune the filter over a broader range without becoming an oscillator.
Another general implementation is shown in FIG. 4 wherein the feedback delay element is replaced by a phase shifter 116, the phase shifter implementing control of the feedback. Signal time delay and signal phase shift are approximately analogous for narrow bandpass filters.
The circuit topology of FIG. 4 is essentially that of the super-regenerative amplifier filter that was developed back in the 1930's (Armstrong). If the resonator 106 is based on a single inductor then the feedback results in a Q-enhanced inductor circuit. If a capacitor is placed in parallel with the Q-enhanced inductor then a tunable filter circuit results. Such circuits are published and well known.
The teachings in United States pre-grant publication no. 2013/0065542 (Proudkii) entitled “Spectral Filtering Systems” are based generally on the circuit of FIG. 4 with a fixed resonator element at low Q, often referred to as a comb-line filter.